Abstract
Cold (few eV) ions of ionospheric origin are widely observed in the lobe region of Earth’s magnetotail and can enter the ion jet region after magnetic reconnection is triggered in the magnetotail. Here, we investigate a magnetotail crossing with cold ions in one tailward and two earthward ion jets observed by the Magnetospheric Multiscale (MMS) constellation of spacecraft. Cold ions co-existing with hot plasma-sheet ions form types of ion velocity distribution functions (VDFs) in the three jets. In one earthward jet, MMS observe cold-ion beams with large velocities parallel to the magnetic fields, and we perform quantitative analysis on the ion VDFs in this jet. The cold ions, together with the hot ions, are reconnection outflow ions and are a minor population in terms of number density inside this jet. The average bulk speed of the cold-ion beams is approximately 38% larger than that of the hot plasma-sheet ions. The cold-ion beams inside the explored jet are about one order of magnitude colder than the hot plasma-sheet ions. These cold-ion beams could be accelerated by the Hall electric field in the cold ion diffusion region and the shrinking magnetic field lines through the Fermi effect.
Highlights
Cold ions of ionospheric origin are widely present in the Earth’s magnetosphere (Chappell et al, 1980; Moore et al, 1997; Cully et al, 2003)
Various types of ion velocity distribution functions (VDFs) with cold-ion beams are observed inside a magnetic reconnection jet, and plasma partial moments of the cold ions are computed to investigate their dynamics through the reconnection process
After a detailed survey of all the ion VDFs, we find four types of ion VDFs inside the jet shown in Figure 3: Type I consists of only a single hot plasma-sheet population, Type II consists of a hot population and a cold population with a positive VB, Type III consists of a hot population and a cold population with a negative VB, and Type IV consists of a hot population with counter-streaming cold populations
Summary
Cold (few eV) ions of ionospheric origin are widely present in the Earth’s magnetosphere (Chappell et al, 1980; Moore et al, 1997; Cully et al, 2003). One prominent feature is that cold ions create highly structured ion VDFs far from the Maxwellian shape Among these distributions, counter-streaming cold-ion beams along the direction normal to the current sheet are frequently observed in the ion diffusion region both in observations and simulations (Fujimoto et al, 1996; Hoshino et al, 1998; Nagai et al, 1998; Shay et al, 1998; Wygant et al, 2005; Divin et al, 2016; Dai et al, 2021). The electric potential well across the separatrix is B2X/2qeN0μ0, where BX and N0 are the reconnection magnetic field and plasma number density in the inflow, qe is the unit of charge and μ0 is the magnetic permeability of free space (Wygant et al, 2005; Divin et al, 2016; Zaitsev et al, 2021) This electric potential can accelerate cold ions to the inflow Alfvén speed VA BX/. Various types of ion VDFs with cold-ion beams are observed inside a magnetic reconnection jet, and plasma partial moments of the cold ions are computed to investigate their dynamics through the reconnection process
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